The present invention relates generally to a time division multiplexed (TDM) transmission system, and more specifically to a phase alignment circuit for a stuff-synchronized TDM transmission system having a cross-connect function.
Pulse stuffing is a well known technique for synchronizing asynchronous lower-level dignal signals before multiplexing them into a higher-level outgoing multiplex signal. This is done by storing the input data streams into a memory and reading it at a nominal bit rate so that all input streams have equal bit rates. According to "positive-zero-negative" stuffing technique, the outgoing data stream has a special time slot known as V-slot which initially contains dummy bits/bytes assigned respectively to the lower-level incoming signals. If the bit rate of a given incoming signal is lower than the nominal rate, the dummy bit of the given incoming signal in the V-slot of the outgoing signal is replaced with a stuff bit/byte (positive stuffing), and if the former is higher than the latter, the dummy bit/byte of the incoming signal is removed from the V-slot (negative stuffing). If the incoming bit rate is equal to the nominal rate, zero stuffing is effected by simply leaving the associated dummy bit/byte in the V slot. The higher-level multiplex signal further contains stuffing specification bits which indicate whether each bit/byte in the V slot is a positive or negative stuff bit/byte, or a dummy bit/byte.
There is often a need to decompose a higher-level multiplex signal into channel component signals for switching to a desired route by interchanging their time slots using a cross-connect, or time division switch. However, as multiplex signals pass through several TDM hierarchical stages during transmission, new stuff bits/byte may be inserted. This often results in the individual channels developing timing discrepancies from a reference phase. To permit the time division switch to properly effect time slot interchange, the phase differences of the individual channels of a frame sequence need to be aligned to the reference phase. As illustrated in FIG. 1, it is the current practice to decompose a frame into individual channels by a TDM demultiplexer 1 for coupling to memories or timing adjustment circuits 2-1.about.2-n and extract stuffing specification bits (SSB) from the frame with a detector 3. Timing adjustment circuits 2 perform timing adjustments on the decomposed channels in accordance with timing information obtained by the extracted stuffing specification bits. The outputs of the timing adjustment circuits 2 are multiplexed again by a TDM multiplexer 4 and supplied to the time division switch where it is further decomposed into channels to be switched to a desired route. Since timing adjustment circuits 2 must be provided in a one-to-one correspondence with the channels of a frame, the total cost and size of the timing adjustment system increase in proportion to the channels multiplexed in the frame.